Figure 9. Sound pressure level produced when emptying hopper barge at the placement site and when dredging in soft sediment as measured over ambient conditions in Cook Inlet, Alaska

ERDC TN-DOER-E14

August 2001

a. Emptying hopper barge at the placement site

b. Dredging in soft sediment

Figure 9. Sound pressure level produced when emptying hopper barge at the placement site and when

dredging in soft sediment as measured over ambient conditions in Cook Inlet, Alaska

clear oceanic waters. For example, Richards, Heathershaw, and Thorne (1996) reported that

concentrations on the order of 20 mg/L could cause an attenuation of 3 dB over a path length of 100 m

at 100 kHz. Although water samples were not taken during this study, suspended sediment

concentrations greatly surpass these levels in Cook Inlet. Thus site-specific conditions should be

an important consideration in environmental assessments of dredging-related underwater noise

elsewhere.

Miles, Malme, and Richardson (1987) and Miles et al. (1986) reported that the loudest sounds

measured in their study were produced during the winching of the loaded bucket up through the

water column. In contrast, the winching events in data collected for this technical note were

relatively weak in terms of acoustic energy compared with that of the bucket striking the bottom

event. This variability indicates that the condition of the dredge plant greatly affects the character

of the sounds produced. Poorly maintained or lubricated mechanical gear can potentially generate

very intense sounds.

The character of bucket dredging sounds also appears to be greatly influenced by the granulometry

of the sediments being dredged; i.e., a bucket impacting coarse sands and gravels, as exemplified

by the dredge Viking performing deepening work in coarse sand and gravel, produced very different,

less intense sounds from those of the dredge Crystal Gayle performing maintenance work in

unconsolidated mud.